Makefile: Ensure initrd parameters in /chosen are up-to-date

The initrd parameters in the FDT /chosen node are fixed when the device
tree blob is generated, so it is important to regenerate the device
tree if the initrd image has been updated.

Currently this does not happen, so the parameters can be stale.
In particular, if the initrd image is bigger than it was when the
device tree was first generated then the value of the linux,initrd-end
parameter is too small, leading to truncation of the initrd image on
boot even though the whole image is present in linux-system.axf.

Make $(FILESYSTEM) a dependency of fdt.dtb so that the parameters are
refreshed properly based on the current initrd image (if any).

Without --with-initrd, FILESYSTEM is the empty string and the
dependency evaporates, which is just fine.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Link: https://lore.kernel.org/r/20250512150944.38322-1-Dave.Martin@arm.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>